Experimental otter trawling on a sandy bottom ecosystem of the Grand Banksof Newfoundland: analysis of trawl bycatch and effects on epifauna

An experimental study of the effects of otter trawling was conducted in a d eep (120 to 146 m) sandy bottom ecosystem of the Grand Banks of Newfoundlan d from 1993 to 1995. Each year, three 13 km long corridors were trawled 12 times within 31 to 34 h with an Engel 145 otter trawl equipped with rockhop per foot gear. The width of the disturbance zones created was on the order of 120 to 250 m. The total biomass of invertebrate bycatch in the trawl dec reased significantly over the 12 sets, even though only a very small propor tion of the biomass present was removed and each set did not pass over exac tly the same area of seabed. An influx of scavenging snow crabs Chionoecete s opilio into the trawled corridors was observed after the first 6 sets (ap proximately 10 to 12 h). Benthic organ isms in trawled and nearby reference corridors were sampled with an epibenthic sled. Their biomass was on avera ge 24% lower in trawled corridors than in reference corridors. At the speci es level, this biomass difference was significant for snow crabs C, opilio, sand dollars Echinarachnius parma, brittle stars Ophiura sarsi, sea urchin s Strongylocentrotus pallidus and soft corals Gersemia sp. The reduced biom ass of epibenthic organisms in trawled corridors is thought to be due to se veral interacting factors including direct removal by the trawl, mortality, damage, predation and migration. The homogeneity of the macro-invertebrate community collected by epibenthic sled was lower in trawled corridors. San d dollars, brittle stars and sea urchins demonstrated significant levels of damage from trawling. The mean individual biomass of epibenthic organisms was lower in trawled corridors suggesting size specific impacts of trawling , especially for sand dollars. No significant effect of trawling was observ ed in the 4 dominant mollusc species captured by the sled (Astarte borealis , Margarites sordidus, Clinocardium ciliatum and Cyclocardia novangliae). T his experiment indicates that otter trawling on a sandy bottom ecosystem ca n produce detectable changes on both benthic habitat and communities, in pa rticular a significant reduction in the biomass of large epibenthic fauna.